This invention relates to a detector circuit to be used in current measurements. The circuit comprises two identically wound toroidal transformers, in which a main current, the primary current, induces magneto-motive forces. These forces are in turn neutralized by other magneto-motive forces induced therein by a compensation current, the secondary current. The detector circuit further includes sensing windings wound on each toroidal core, at least one of which sensing windings carries a magnetizing current, the sensing current, from an external current source. The sensing current is conducted to a synchronous rectifier to provide a d.c. control signal for the compensation current.
From the literature, cf e.g., ETZ, Vol 100, 1979, No. 24, pp 1390-94, a zero-flux current transformer is known which is used for the measurement of the current supplied to an electromagnet in a particle accelerator. Since very heavy currents of several hundred amperes are involved, it is appropriate to convert the main (primary) current into a more handy measuring current of a reduced intensity and conduct this current through a measuring resistor. The voltage drop over this measuring resistor is then used as a measure of the intensity of the main current.
The prior art zero-flux current transformers consist in a combination of a magnetic integrator and a 2nd-harmonic magnetic modulator.
The magnetic integrator is in principle comprised by a ring-shaped or toroidal core of a ferromagnetic material having a primary winding, a secondary winding and a sensing or indicating winding. The latter winding is connected to the input terminals of an amplifier, which drives the secondary current through a measuring resistor. Any change of the magnetic flux in the toroid will induce a voltage across the sensing winding, which voltage will influence the amplifier to provide a compensation current to neutralize the change of flux provided by the primary current. The magneto-motive forces induced by the current through the primary winding will thus be outbalanced that is, cancelled out by the magneto-motive forces induced by the current through the secondary winding, so that there exists a certain ratio between the current intensities in the primary and the secondary windings.
However, offset voltages occurring in practice and the fact that the obtainable amplification in the feed-back loop is not infinite, imply that a small voltage is induced in the sensing winding, which inevitably results in a gradual increase of the flux until the core is satunated. In order to stabilize the flux at a zero value, a further toroid is used, which is as identical as possible to the first one and which operates as a 2nd-harmonic modulator sensing any possible unbalances between the magneto-motive forces on the primary and on the secondary side. By a perfect balance, the magnetizing current through the sensing winding on said further toroid is symmetrical and therefore contains odd-harmonic components only, and the output signal from the synchronous rectifier, cf. the article, is zero. If an unbalance occurs between the induced magneto-motive forces, the mean flux will differ from zero, resulting in an asymmetric magnetizing current, which further contains even-harmonic components, of which the 2nd-harmonic is by far the dominant one. Its amplitude and phase depends on the magnitude and sign of the unbalance. By detection of this 2nd harmonic, there is derived a d.c. signal corresponding to the unbalance, which signal is used to control the amplifier in the secondary circuit in such a way that the balance between the magneto-motive forces induced by the primary and secondary currents is reestablished.
The most serious disadvantage of this circuit is that it is very costly in components. Just the employed 2nd-harmonic bandpass filter, which is an active filter to treat a very composite signal containing a large number of even- and odd-harmonic components, needs a large amount of circuitry.